In an HVDC system, two or more HVDC converter stations are connected by means of one or more HVDC lines, where an HVDC line is used for carrying a high voltage direct current from one HVDC converter station to another. In monopolar HVDC transmission, a single HVDC line connects two HVDC converter stations, and the current can be returned via earth. In bipolar HVDC transmission, two HVDC converter stations are connected by means of two HVDC lines. The current can hence be returned via an HVDC line, and in normal operation, there is no unbalanced earth or neutral current. However, if an outage occurs in one of the converters in a bipolar system, the current must be returned via earth, or via a neutral conductor.
In order to avoid that any unbalanced current causes unacceptable interference or corrosion, the part of an HVDC converter station having the potential closest to ground, often referred to as station neutral, is typically connected to a remotely located ground electrode via an electrode line or to the station neutral of another HVDC converter station via a metallic return conductor between two HVDC converter stations. In the latter case, the station neutral of both HVDC converter stations are connected to earth at one point only, which may for example be the earth grid of one of the HVDC converter stations, and hence, there is no earth current.
The operation voltage of an electrode line is low. Typically, the operating voltage of an electrode line at rated current in the HVDC line(s) is a few kV, while the operating voltage of a metallic return conductor is often larger, since a metallic return conductor is typically longer. In a bipolar HVDC system, the electrode line current, and hence the electrode line voltage, is nil at balanced operation. Hence, since a fault does not imply any significant reduction of the electrode line voltage, any fault on an electrode line could remain undetected for a long time, unless special measures are taken.
The length of an electrode line is often 20 kilometers or more, in order to ensure that any strong currents will be led to a geographical position where injection of the line current in the earth electrode will cause no or little interference with the HVDC station or with the surroundings, for example by way of corrosion. The grounded electrode of an electrode line is typically located at a geographical position having low resistance to remote earth (e.g. to the earth's magma), and which is at a sufficient distance from any towns, underground pipes, railroads etc.
In the following, the term electrode line will be used to refer to a metallic return conductor, as well as to an electrode line connecting the DC station neutral of an HVDC station to a ground electrode.
An electrode line with an earth fault could cause a safety hazard, since currents being led via the electrode line would then be led to an uncontrolled geographical position, possibly harming humans as well causing corrosion on local infrastructure. Furthermore, in case of a pole outage in a bipolar system, the remaining pole will typically also have to be tripped if the electrode line has a ground fault, in order to clear the electrode line ground fault. Such outages of both poles are very costly, both for society in general and for the HVDC line operator. Also under normal operating conditions, an electrode line with an earth fault could cause damage to the surroundings, for example corrosion in underground pipes.
Hence, means for monitoring electrode lines in an HVDC system in order to detect faults, such as ground faults or short circuits, are desired. Furthermore, since the electrode lines typically extend over 10's of kilometers, it is often desirable not only to detect the presence of a line fault, but also to determine the approximate location of the fault, so that the fault can be easily and quickly attended to.
A method of detecting a faulty electrode line composed of two leads in a bipolar HVDC system is disclosed in U.S. Pat. No. 6,518,719. A balanced-to-ground pulse is formed from an unbalanced-to-ground pulse in a push/pull mode by use of a transformer having two high voltage windings and a low voltage winding, the low voltage winding being connected to an output of a pulse generator. The pulse is then fed into the two electrode leads, and an echo curve is recorded and compared to a target echo curve. The points where the pulse enters the electrode leads are located at a distance corresponding to λ/4 from the branch point, i.e. the point where the electrode line branches off into the two leads, λ being the wavelength of the centre frequency of the feed pulse. A fault signal is generated when a tolerance band placed around the echo difference curve is exceeded.